Process for purifying sulphur dioxide bearing gases



H in which sulphur is present as sulphur dioxide. free of sulphur compounds without discriminat- 5 .10 reduction of the sulphur dioxide to elemental sul- 2H2S+SO292H20+3S 25 LamoreauxU. S. Patent 1,182,915. However, hydrogen sulphide in the exit gas so that, in cf- 1 ceed as simply as it might to produce elemental the gas stream.

.45 original gas. That these reactions occur with other side'reactions with sulphur occur to any 45 PATENT OFFICE PROCESS FOR PUR IFYING SULPHUR DIOX- llDE BEARING GASES John W. Beckman, Oakland, Calif.

No Drawing. Application June 25, 1932, Serial No. 19,277

1 Claim. (Cl. 232) This is a continuation in part of my application sulphur dioxide, could be prevented or minimized. Serial No. 379,994, July 22, 1929. Thus, gases from sources as coal containing sul- This invention is concerned with the purificaphur compounds have been proposed as suitable tion of gases such as are created in industry and reducing agents along with other gases possibly Ni-TE@ ST l The discharge of sulphur dioxide into the atmosing between the utility of the gases and the great phere causes much damage to property and life difference in final results. and has been the subject of much litigation. While hydrogen sulphide and sulphur dioxide While many processes have been proposed for react phur, as will presently appear, industrially none of these have been successful, probably because the rate is very low particularly at those temperthey accomplished nought but to convert some atures favorable to Equation 1. In fact the lower of the sulphur from the form of S02 into H2S, a the temperature, the more satisfactory does the material whose presence in the atmosphere is alrate of the above reaction become, although it is 15 most as objectionable as that of S02. The pronot rapid and high pressure and violent agitation duction of H28 is obviated upon reducing S02 in is depended upon by Bacon in U. S. Patent 1,807,- accordance with this invention and with the type 583 to secure results. Once hydrogen sulphide is reaction: formed upon reduction of the sulphur dioxide, its

elimination is comparatively diflicult as it is not easily oxidized. Production of elemental sulphur by the reduc- I have found that it is possible to reduce sultion of sulphur dioxide has previously been prophur dioxide substantially completely to eleposed by SmithU. S. Patent 878,569 and by mental sulphur without having substantially any these proposals, and numerous others, have never feet, the type reaction (Equation 1) is followed been adopted to any extent by industries exand goes completely to the right. This eliminates hausting sulphur dioxide to the atmosphere as any problem as to handling the hydrogen sulore roasters. While definite reasons for this canphide or other sulphides, as COS and CS2 and not, obviously, be given, it is believed to depend substantially the only treatment required for the 30 on the fact that the type reaction does not proexit gas is to remove the elemental sulphur from sulphur and because it is accompanied by others, In accordance with my invention the reducpossibly the following which create hydrogen suling' gas or atmosphere is free of sulphur or sulphide and other materials: phur products, particularly hydrogen sulphide.

Thus, sulphur free carbon monoxide, hydrogen, 35 methane, ethane, natural gas, vaporized hydrocarbons, and other reducing fluids free of sulphur E gggjgg g in-elemental or combined'form can be used suc- 40 3S 4H il 280 cessfully. The term non-sulphur bearing is 40 2 2 2 usedto refer to such reducing materials. The

If the final exit gas is contaminated with hyreducing gas is intermixed with the sulphur didrogen sulphide, a problem is presented which is oxide or sulphur dioxide carrying gas and realmost as serious, if not as serious, as With the acted at a temperature below that at which that in Equation 1 is recognized for Benner in appreciable extent- I have found 600 C. gen- U. S. Patent 1,741,551 has suggested the reaction erally satisfactory although some latitude is posof the resulting hydrogen sulphide with a special sible as will be indicated later. The reacting catalyst at a temperature below about 200 C. gases are substantially dry and free of water But heretofore, as far as I am aware, it has not to avoid reactions of the type of Equation 3, while 50 been realized and pointed out that, by controlling the more stable forms of hydrocarbons not subthe chemical nature of the reducing gas, its physject to decomposition at the temperature and ical condition, the quantities reacted and the pressure used to give hydrogen are preferred temperature, the formation of hydrogen sulphide, since the possibility for the reaction in Equation 2, and other sulphur compounds, upon reduction of to occur is minimized and control is easier. 55

The sulphur dioxide gas can be in diluted form -mixed with nitrogen and other gas as when drawn from a sulphide ore roasting furnace. It can be concentrated by any of the well known means as by absorption in water so as to rid it of other gases, the water being heated, all as disclosed in British Patent No. 6,404 of 1885,- t8 liberate the sulphur dioxide. Concentration of the sulphur dioxide is preferred since less gas has to be handled and it is possible to rid the sulphur dioxide of hydrogen sulphide and moisture before reducing it to secure elemental sulphur. I

The reducing gas is freed of water or moisture by cooling and condensing out the. water or by the use of a desiccant. The reducing gas, if not initially free, can be rid of sulphur by any of the known processes. Thus, a hydrogen sulphide contaminated natural gas can be purified by scrubbing with a sodium carbonate-bicarbonate solution, the sulphur from the hydrogen sulphide being liberated in elemental form and the solution renewed upon blowing it with air in the presence of NiS or CoS as a catalyst. The par ticular details of. freeing the reducing gas of sulphur or moisture form no part of this invention and any known practises can be used to practise these steps.

In practising the invention I have used substantially sulphur free, dry CO, passing two parts of the gas to one of S02 through a chamber filled with shredded asbestos, broken porous brick, glass sand or lump lime or other catalytic material. The gas mixture in contact with the asbestos or other material was kept at a temperature of about 400 C. I have found temperatures between 350 C. and 850 C. best; as a rule as the temperature is raised the conversion efiiciency and production of sulphur falls off and the reactions shown by Equations 2 to 6 inclusive take place, particularly approaching and beyond 700 C. The range between 400 C. and 700 C. is, I have found, most satisfactory and maximum conversions took place between these temperatures at about 600 C. After passing over the shredded asbestos, or other material, the mixture was treated to precipitate the elemental sulphur as by settling, precipitation and filtration.

In another process successfully conducted I passed a mixture consisting of substantially sulphur free, water free natural gas and sulphur dioxide in a volume relationship of one to two over shredded asbestos maintained at about 400-500 C. A dense cloud of yellow sulphur was emitted from the heated chamber containing the asbestos. Silica glass sand (99.6% S102) gave the same result although this catalyst operated better with the chamber at the higher temperature of 520 C. Diatomaceous earth (crushed Sil-O-Cel, having a fineness of from a quarter inch mesh down, and a composition of 92% SiOz, 0.6% CaO, 0.7% MgO, 3% A1203), gave successful results also at a temperature of 400 C. The natural gas used was almost all methane, dry and substantially sulphur free.

The material with which the gas mixture is contacted is a catalyst since it remains unchanged and is present in its original form after the reaction is over. It is to be noted that the catalytic materials disclosed, broken brick, shredded asbestos, diatomaceous earth, silica glass sand, porous lump lime and silica gel, are all of a very porous nature having a large surface per unit volume and to some extent refractory. Thus, silica gel, it is estimated, has an area of an acre for each cubic inch of material. Since it is the surface effect of the materials that has the effect, the chemical constituency of the materials does not enter in as far as the catalysis is concerned. Thus these materials are inert with respect to the sulphur dioxide and reducing fluids employed, and they do not enter into combination with the sulphur to form sulphides or sulfites. It has been previously disclosed in Bulletin 84 of the Bureau of Mines, Metallurgical Smoke, page '72 et seq. that sulphides of barium and calcium are reversion reagents useful for the reduction of sulphur dioxide at temperatures above 750 C. See also Young Patent 1,094,767. However, I have found lump lime of a porous nature to be a surface catalyst also at a lower temperature than that mentioned by Young and that between 400-600 C. HzS formation does not occur although it will if the temperature is raised to 700 C. and higher as Young teaches. This use of porous lime is to be distinguished from that use of calcium sulphide or uses of lime under conditions where the lime is immediately converted to the known reversible sulphides. Under the conditions of my process, lime is useful by reason of its porosity and surface eifect as are the other materials.

The proportion of sulphur dioxide to reducing gas should be controlled so that the minimum, if any, of any undesirable reaction products is formed. The proper proportions can be determined from a consideration of the equations of the reaction with sulphur dioxide for the particular reducing gas used. Too much of the reducing fluid may result in too large a formation of water or other undesirable so that side reactions can occur and create unwished for hydrogen sulphide or other products.

The preferred conditions include the use of a. dry and non-sulphur bearing" reducing gas and sulphur dioxide gas freed of moisture and other sulphur compounds. Under these conditions the conversion is practically complete. Of course, non-observance of any one of these conditions results in a lower efficiency, approximately in proportion to the lack of observance of the desired conditions. In practise I have reduced sulphur dioxide so rapidly that hydrogen sulphide was substantially absent from the exit gas while practically of the sulphur dioxide was reduced to'free, recoverable sulphur. I find that the temperature has much to do with HzS formation and that temperatures between 400 and 600 C. were the most satisfactory, and practically quantitative yields of sulphur have been secured at 400 C. or close thereto.

By the term non-sulphur bearing I do not wish to exclude such gases as CO, CH4, H2, natural gas, etc., which contain an impurity of sulphur as HzS. The absence of sulphur impurities from both the reducing gas and the S02, as well as the absence of moisture, is desirable but is not an absolute essential. The most essential things are (1) observation of the temperature and keeping it at a point where I-IzS formation is a minimum and (2) use of non-sulphurous reducing gas as CO or H2. The other factors I have mentioned are not essential absolutely to secure good results and I have operated successfully without paying any attention to them.

i claimi A purification process for eliminating sulphur from a gas containing sulphur substantially only in the form of sulphur dioxide so that after said sulphur has been removed said gas can be passed into the atmosphere, the process comprising passing into contact with a porous lime at a temperature of about 400 C. a substantially dry gas containing sulphur substantially only in the form of sulphur dioxide and a reducing agent for the sulphur dioxide whereby said sulphur dioxide is reduced to elemental sulphur and the gas remaining is free substantially of sulphur in any form except said elemental sulphur, then removing said elemental sulphur from said remaining gas, and then releasing said remaining gas to the atmosphere.

JOHN W. BECKMAN. 

